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Section 12.1 – Structure of DNA DNA – Deoxyribonucleic Acid; traits are determined by your genes, genes code for proteins, and genes are coded for in your DNA *Was discovered in the 1940’s to be the genetic material responsible for traits.

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I.Scientists Discovering DNA A.Frederick Griffith (1928) 1.Worked with two strains of a pneumonia bacterium, one pathogenic and one harmless 2.When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic 3.Transformation – one strain of bacteria is changed into another strain; some “factor” was transferred from one strain to the other

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C.Alfred Hershey & Martha Chase (1952) 1.Provided further evidence that DNA is genetic material by using bacteriophages (phages), viruses that infect bacteria 2.Used a phage known as T2 which infects E. coli 3.Radioactive Isotopes – used to tag DNA and protein Sulfur to tag protein Phosphorus to tab DNA 4.DNA was proven to be the genetic material of viruses versus protein

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I.The Shape of DNA A.A Winding Staircase 1.DNA is made of 2 parallel strands linked together and shaped like a spiral staircase. 2.The spiral shape is called a double helix. 3.Each strand is made of linked subunits called nucleotides.

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B.Base-Pairing Rules (Chargaff’s Rules) 1.A Purine on one strand is always paired with a Pyrimidine on the other strand. 2.More specifically: Adenine with Thymine (A with T) Cytosine with Guanine (C with G) 3.The base pairs are held together by weak hydrogen bonds and are called complementary base pairs.

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C.Complementary Strands 1.The two strands are complementary because they fit together like a puzzle and their bases are identical. Ex: One strand has the following nucleotides: ATTCGGTACCCC The other strand would be what? TAAGCCATGGGG

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III.Discovering DNA’s Structure A.Edwin Chargaff – in all organisms, the amount of adenine almost equaled the amount of thymine, and the amount of cytosine equaled the amount of guanine. Base-Pairing Rule!!

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C.James Watson and Francis Crick 1.Used the findings of Chargaff and Franklin, and their knowledge of chemical bonding to build an accurate, three-dimensional model of DNA 2.They were also able to propose a method of DNA replication based on its structure.